This invention relates to a method for controlling the oxygen content of agglomerated molybdenum powders by controlled oxidation of the powders. More particularly it relates to a method of introducing a controlled amount of oxygen into agglomerated molybdenum powders by heating the powders in the presence of water vapor and a controlled relative amount of a non-oxidizing atmosphere.
Flame spraying and plasma spraying are common techniques for the application of protective and wear resistant coatings of various metals, ceramics, and cermets, usually to metal surfaces (substrates). The piston ring industry commonly uses molybdenum coatings on rings for internal combustion engines.
In the flame spraying technique an electric arc or an oxyacetylene flame melts the end of a continuous coil of molybdenum wire and a gas propels it onto a substrate for example, the wear surface of a cast iron piston ring where it splats and solidifies, forming the coating in successive layers. Because of the presence of excess oxygen either from the flame of the surrounding air, or both, the coatings produced by this technique contain large quantities of oxygen, typically from about 7% to about 8% in solution and as various molybdenum oxides. The large quantities of oxygen in the molybdenum apparently harden the coating.
In the plasma spraying of molybdenum, there is usually a minimum of oxygen in the sprayed coating due to the use of an oxygen-lean plasma gas system. That is, argon, helium, hydrogen, nitrogen, or combinations of these gases, all of which are relatively free from oxygen, are used in the plasma spraying process. Hence, any oxygen in the sprayed coating is incidentally due to oxidation of the molten particles by oxygen impurity in the plasma gas and/or surface oxidation of the freshly deposited coating. In such "pure" molybdenum coatings the oxygen level is in the 1% to 2% range. Such coatings are softer than their flame sprayed counterparts.
For higher hardness, therefore, a more expensive process such as the flame spray process which requires wire, or a more expensive powder such as molybdenum plus nickel-base alloy must be used.
It would be desirable therefore to have a method of producing molybdenum powders of sufficiently high oxygen content to enable them to be used in a plasma spray process to produce hard coatings.
U.S. Pat. No. 4,146,388 describes and claims molybdenum plasma spray powders and a process for producing the powders of molybdenum and oxides of molybdenum having an oxygen content of from about 0.5 to about 15% by weight oxygen. The process involves passing molybdenum particles through a plasma with oxygen or oxides of molybdenum to produce the oxygen containing powder.